This invention relates to low alloy precipitation hardened steels characterized by high strength and toughness in plate form. More particularly, this invention relates to copper precipitation hardened low alloy Ni-Cu-Cb steels having high strength, improved low temperature toughness, especially in thick sections, and excellent weldability resulting from low carbon and in some embodiments low sulfur content.
In the 1960's, the plate steels available for structural applications were either as-hot rolled C-Mn-Si steels with ferrite-pearlite (F-P) microstructures or quenched and tempered (QT) low alloy steels with tempered martensitic microstructures. The F-P steels exhibited yield strengths on the order of 60 ksi (413.7 MPa) while the QT steels had yield strengths in excess of 80 ksi (561.6 MPa). In both the F-P and the QT steels, the strength of the steel is derived from the carbon content. Unfortunately, while the carbon content is effective in imparting strength, it is also responsible for lower weldability and weldment properties.
One family of QT steels which is used in Navy ship and submarine construction is known as the HY (High Yield strength) steels; HY-80, HY-100, HY-130 and HY-180. Of particular interest here is HY-100 which has an allowable yield strength range of 100-120 ksi (689.5-827.4 MPa). A typical composition (wt.%) of HY-100 is 0.15 C-0.3 Mn-3.0 Ni-1.4 Cr-0.4 Mo. This steel is capable of being hardened by the QT treatment in section thicknesses in excess of 4 inches (10.2 cm). After the QT treatment, HY-100 can exhibit adequate levels of strength and toughness in the base plate. However, the combination of the relatively high carbon content and overall alloy content render this steel prone to hydrogen-related cracking in the heat-affected zone (HAZ) which develops during multi-pass welding of the plates. The problem of hydrogen-related HAZ cracking could be avoided only through the use of very expensive weld process controls such as preheating and restricted welding conditions.
To overcome such problems, high strength low alloy steels were developed having potentially the same or better strength and toughness properties and with a more easily welded microstructure. Generally, such steels were obtained by a combination of "clean" steel processing, controlled microalloying elements, and heat treatments. For example, U.S. Pat. No. 3,692,514, issued Sep. 19, 1972, discloses a low alloy steel adapted for structural and line pipe use. Such steel included alloying small amounts of columbium, vanadium, titanium, aluminum, boron, and nitrogen which functions in grain refinement and precipitation hardening to increase strength and toughness in a conventional carbon-manganese structural grade steel. Further increases in strength were achieved with nominal amounts of conventional alloying of copper, cobalt, nickel, and molybdenum along with proper heat treatment. Such steels exhibited improved weldability with less stringent controls such as eliminating preheating prior to welding. Although such steels were developed for line pipe, they also found applications in valves and fittings for pumping stations, off-shore platforms, oil well servicing equipment, large mining and off highway trucks, and also as structural plate for naval ships and submarine construction.
The use of such steels and the weldments in ships and vessels was directly related to the extraordinary toughness, high strength and deformation performance under high rate loading. Ordinarily used for general construction, such ferritic steels having high weldability were found suitable for ship construction. Welding of ships and vessels, however, is a very labor intensive process due principally to the design requirements of water tight integrity, compartmentation, and shock resistance, for example, and the difficulty of using automated welding in the majority of ship construction. A key advantage of high strength low alloy steels is the inherent weldability and the attendant lack of preheat requirement as part of the welding process. Such improved weldability directly translates into significant fabrication cost reductions while at the same time providing weight savings which can be achieved by the substitution of high strength steels in small cross sections. Such improved weldability is primarily attributed to the low carbon content, generally on the order of 0.04 to 0.08%. The problems, considerations and development work associated with higher strength steel plates for ship and vessel construction are described in the Journal of Ship Production, Vol. 2, No. 3, August 1986, pages 145-162.
The literature also contains many references concerning the influence of steel composition on final properties. In general it is well recognized that toughness is improved by lowering the amounts of certain elements contained in the steel, especially carbon and sulfur. A dramatic improvement in toughness with lower carbon levels of less than 0.04% is described in a paper entitled "Structure, Hardenability and Toughness of Low-Carbon High-Strength Steels" by McEvily, et al. reported in the symposium Feb. 27-28, 1967, "Transformation and Hardenability in Steels." The high yield strength (greater than 100 ksi) steels, nominally containing 3% Ni-3% Mo and 0.7% Mn, also exhibited high toughness in the as-rolled condition without further heat treatment to develop the strength and low temperature toughness.
There is ample information in the literature that both cold cracking resistance and HAZ toughness are reduced with increased carbon content. In FIG. 1 of an article by B.A. Graville, in Proceedings Welding of HSLA-Microalloy Structural Steels, ASM, 1978, pp. 85-101, susceptibility varied with both carbon content and carbon equivalent value. It was also noted that carbon content had the most important influence can susceptibility to cold cracking. Haze et al (T. Haze, S. Aihara and H. Mabuchi, in Proceedings Accelerated Cooling of Rolled Steel, Canadian Inst. Mining and Metallurgy, Pergammon, 1988, pp. 235-248) have shown in FIG. 2 that the low temperature toughness of the coarse-grained HAZ was reduced as the amount of high carbon martensite islands increased. During the intercritical annealing which occurs inadvertently in multi-pass welding, the amount of martensite which is present after cooling would increase linearly with the carbon content.
Lower carbon contents ranging from 0.01 to 0.03% in ultralow carbon bainitic (ULCB) steel have also been shown to improve weldability and toughness. A paper entitled "Development of Controlled Rolled Ultralow Carbon Bainitic Steel For Large Diameter Line Pipe" by H. Nakasugi, et al. from Alloys for the 80's, pages 13-14, discloses such a steel containing 1.5 to 2.0% manganese as well as nominally 0.04 niobium to achieve the strength and toughness in the steel. Such steels would not be suitable for ship plate because of strength and thickness L0 limitations.
The influence of the sulfur level on notch toughness is also well recognized at high temperatures where the upper shelf toughness is increased with lower sulfur.
What is needed is a high strength low alloy steel which can achieve mechanical properties which fall within prescribed narrow ranges, e.g., 100.ltoreq.YS.ltoreq.120 ksi (689.5 .ltoreq.YS.ltoreq.827.4 MPa) and Charpy V-notch toughness CVN .gtoreq.35 ft. lbs. at -120.degree. F. (-84.degree. C.) for a wide range of plate thicknesses, e.g., 0.5-6.0 inches (1.3-15.2 cm). Furthermore, it is desirable that this steel be producable by straightforward steelmaking techniques. Still further, it is desirable that this steel exhibit improved weldability over a wide range of welding conditions, e.g., heat inputs and plate thicknesses. In addition, the weldments in this steel should be immune to cold and hydrogen cracking without preheating. Still further, the new steel should exhibit good strength and toughness in the HAZ.